Experimental evidence of interface resonance states in single-crystal magnetic tunnel junctions

All solid-state tunnel spectroscopy experiments performed on single-crystal Fe/MgO/Fe magnetic tunnel junctions show sharp features at 0.2 and 1.1 V. These peaks are observed on the electrical differential conductivity only in the antiparallel magnetic configuration and only for the voltage sign corresponding to the injection of electrons toward the bottom electrode. They are attributed to the conductivity of two different resonant states of the Fe(001)/MgO bottom interface. The analysis of the attenuation of these peaks as a function of the insulator thickness provides information on their symmetry.

Figure 1

(Color online) (Left) Schematic of the experimental setup. (Right) Typical $I\left(V\right)$ curve obtained for a 2.2-nm-thick MgO layer after averaging 100 measurements.

Figure 2

(Color online) $\frac{dI}{dV}\left(V\right)$ for the parallel and the antiparallel Fe magnetization configurations for a 2.6-nm-thick MgO tunnel barrier. Inset: $\left|\frac{d^{2}I}{d{V}^2}\left(V\right)\right|$ for the same states. The peaks at 0.2 and 1.1 V for the antiparallel state are not observed for the parallel state.

Figure 3

(Color online) $\left|\frac{d^{2}I}{d{V}^2}\left(V\right)\right|$ for the antiparallel configuration for different MgO thicknesses. The amplitudes of the two peaks increase as the MgO thickness decreases.

Figure 4

(Color online) Difference between the differential conductivities in the antiparallel and parallel states, $\delta i\left(V\right)=\frac{d\left(I_{\text{ap}}-I_{\text{p}}\right)}{dV}\left(V\right)$, for different MgO thicknesses: 2.2, 2.4, and 2.6 nm. Inset: the 2.4 and 2.6 nm curves have been, respectively, multiplied by attenuation coefficients equal to 4.3 and 14.2. All the curves superimposed showing that neither their position nor their widths are dependent on the MgO thickness.

Figure 5

(Color online) The black (gray) line is the transmission coefficient for $k_{\parallel }=0$ extracted from Ref. 3 for the ${\Delta }_1$ $\left({\Delta }_5\right)$ symmetry. The small triangles are the transmission coefficient ${\alpha }_t$, where $1/{\alpha }_t$ is the coefficient found for superimposing the 2.4 and 2.6 nm curves to the 2.2 nm one (Fig. 4). For these curves the reference is taken for 2.2 nm.